Quantitative Characterization and Mechanism of Formation of Multilength-scale Bulk Heterojunction Structures in Highly Efficient Solution-Processed Small-Molecule Organic Solar Cells

Abstract

In this study we used simultaneous grazingincidence small-and wide-angle X-ray scattering (GISAXS and GIWAXS, respectively) to probe the multilength-scale structures of thin active layers comprising the linear A−D− A-type π-conjugated donor molecule TBDTCNR and the fullerene acceptor molecule PC 61 BM for use in solutionprocessed small-molecule-based organic solar cells (SMOSCs). We found that the pseudo-two-dimensional fractal-like networks in the bulk heterojunction (BHJ) structure were determined by mutual interactions between the small-molecule (SM) crystallites and the nanoscale PC 61 BM clusters during their formation and phase separation, and deduced quantitatively, at multiple length scales, the BHJ structures comprising these SM crystallites and PC 61 BM clusters. We also conducted in situ GIWAXS measurements to study the temporal behavior and kinetics of SM crystallization from solution to the solid film state. Our GISAXS/GIWAXS study revealed that the multilength-scale BHJ structures in the thin films could be tuned effectively by varying the amount of incorporated PC 61 BM and the annealing temperature. This study provides fundamental information relating to the mechanism of formation of hierarchical BHJ structures through relatively rapid crystallization of a highly crystalline SM, as well as the relationships among the hierarchical structure, the photovoltaic performance, and the mechanism of formation, thereby allowing greater control over BHJ structures in SMOSCs with optimized fabrication and performance.

Distinct Photovoltaic Performance of Hierarchical Nanostructures Self-Assembled from Multiblock Copolymers

“…Our results demonstrate that hierarchical nanostructures self-assembled from D′(AD) n A′ multiblock copolymers can improve the photovoltaic performance. Some experimental evidence, supporting these simulation results, is available in the literature. − For example, Yan et al prepared polymer blend solar cells with hierarchical nanostructures. The polymer solar cells were based on blends of the donor polymer poly­(3-hexylthiophene) (P3HT) with the acceptor polymer poly­([ N , N ′-bis­(2-octyldodecyl)-11-naphthalene-1,4,5,8-bis­(dicarboximi-de)-2,6-diyl]- alt -5,5′-(2,2′-12-bithiophene)) (P­(NDI2OD-T2)) .…”

“…The morphology of the active layer is a key factor for high performance. , Optimizing the morphology can enhance the exciton dissociation (charge carrier generation) at the donor/acceptor (D/A) interface, improve charge transport, and maximize the collection of the dissociated electrons and holes at the electrodes. Various structures, such as ordered continuous networks, lamellar, and cylindrical structures, have been applied to optimize the morphology. − Among these structures, hierarchical nanostructures with different length scales have recently been reported to possess improved photovoltaic properties. − Chen et al have fabricated PTB7/fullerene photovoltaic devices harboring hierarchical nanostructures ranging from several nanometers of crystallites to tens of nanometers of nanocrystallite aggregates in PTB7-rich and fullerene-rich domains . These devices exhibit superior performance, which is attributed to the significantly enhanced exciton dissociation resulting from the hierarchical morphologies.…”

Regioisomeric thieno[3,4-d]thiazole-based A-Q-D-Q-A-type NIR acceptors for efficient non-fullerene organic solar cells

“…The orientation of a molecular packing can be correlated to the location of the diffraction peak relative to the substrate. 44,58,59 The (010) π–π stacking are found to be normal to the Si substrate at q z = 1.57–1.78 Å −1 while the (100) lamellar stackings arrange themselves in the plane of Si substrate at q xy = 0.26–0.31 Å −1 . Both blend films exhibit preferential ‘face-on’ orientation.…”

Achieving High Efficiency and Stability in Organic Photovoltaics with a Nanometer-Scale Twin p–i–n Structured Active Layer

“…The GISAXS profiles of other pristine and blend films display characteristics of fractal structure factors (power-law intensity behavior in a specific q range), indicating the formation of a fractal-aggregation network, and this network, therefore, cannot be properly fitted with the cylindrical form factor as described in the previous literature. 33,72 The fractal network model for modeling the morphology of the active layers in OPV was carried out in the literature, 65,73 and it was recently adopted by several groups for the various morphology of the active layers of OPV materials. 7,[62][63][64]74,75 The equations and explanations on modeling of the fractal network can be found in the Supporting Information.…”